Shortening currency list in Configuration->General

use isset

array_flip

app/code/Magento/Config/Model/Config/Source/Locale/Currency.php

@@ -4,12 +4,15 @@
  * See COPYING.txt for license details.
  */
 
-/**
- * Locale currency source
- */
 namespace Magento\Config\Model\Config\Source\Locale;
 
+use Magento\Framework\App\Config\ScopeConfigInterface;
+use Magento\Framework\App\ObjectManager;
+use Magento\Framework\Locale\ListsInterface;
+
 /**
+ * Locale currency source.
+ *
  * @api
  * @since 100.0.2
  */
@@ -21,27 +24,70 @@ class Currency implements \Magento\Framework\Option\ArrayInterface
     protected $_options;
 
     /**
-     * @var \Magento\Framework\Locale\ListsInterface
+     * @var ListsInterface
      */
     protected $_localeLists;
 
     /**
-     * @param \Magento\Framework\Locale\ListsInterface $localeLists
+     * @var ScopeConfigInterface
      */
-    public function __construct(\Magento\Framework\Locale\ListsInterface $localeLists)
-    {
+    private $config;
+
+    /**
+     * @var array
+     */
+    private $installedCurrencies;
+
+    /**
+     * @param ListsInterface $localeLists
+     * @param ScopeConfigInterface $config
+     */
+    public function __construct(
+        ListsInterface $localeLists,
+        ScopeConfigInterface $config = null
+    ) {
         $this->_localeLists = $localeLists;
+        $this->config = $config ?: ObjectManager::getInstance()->get(ScopeConfigInterface::class);
     }
 
     /**
-     * @return array
+     * @inheritdoc
      */
     public function toOptionArray()
     {
         if (!$this->_options) {
             $this->_options = $this->_localeLists->getOptionCurrencies();
         }
-        $options = $this->_options;
+
+        $selected = array_flip($this->getInstalledCurrencies());
+
+        $options = array_filter(
+            $this->_options,
+            function ($option) use ($selected) {
+                return isset($selected[$option['value']]);
+            }
+        );
+
         return $options;
     }
+
+    /**
+     * Retrieve Installed Currencies.
+     *
+     * @return array
+     */
+    private function getInstalledCurrencies()
+    {
+        if (!$this->installedCurrencies) {
+            $this->installedCurrencies = explode(
+                ',',
+                $this->config->getValue(
+                    'system/currency/installed',
+                    \Magento\Store\Model\ScopeInterface::SCOPE_STORE
+                )
+            );
+        }
+
+        return $this->installedCurrencies;
+    }
 }